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Coastal morphodynamics in subsiding areas

In the modelling of coastline evolution due to subsidence and sea level rise it is important to consider the coastal sediment balance for the present conditions. The coastal sediment balance describes the different sources and sinks of sediment into the coastal system as well as the variation of the littoral transport along the coastline. The importance of different process parameters such as the littoral transport, the relative sea level changes, fluvial sediment sources and human interventions must be analysed prior to the simulations of future coastal morphodynamics.

The sediment transport mechanisms and the coastline evolution on the three coastal stretches of Ravenna, Rimini and Cesenatico along the Northern Adriatic which are characterised by different subsidence rates were investigated.

The coastline at Ravenna is highly dominated by human interventions through the years. The delta of the Fiumi Uniti was building out seawards during the end of the last century and the beginning of this century. By this time, the river mouth was located about 400 m north of the present distributor channel. Over the period between 1835 and 1991 a gradual southward migration of the river mouth has taken place. During the first half of this century, the delta started to erode. This erosion was due to changes in hydrodynamic conditions (e.g. waves and currents), a decreased sediment supply from the river or a combination of these.

The coast south of Rimini has at all times been subject to accretion. This is mainly due to a long pier at the Rimini gat which was constructed several hundreds of years ago in order to protect the mouth of the Marecchia river. The sediment accumulation is caused by the blocking effect of the pier on the littoral transport. The pier has been prolonged a number of times. Since the last prolongation in 1923-25 the total length of the pier is approximately 400 m. Since 1942 a large part of the water of the river is being diverted about one kilometre to the north.

Until the late 1960's the coast north of the Cesenatico gat was subject to an accretion of up to 2.5 m per year. Between 1969 and 1972 the pier at Cesenatico was extended by approximately 50 m. Due to this extension the northgoing sediment transport had decreased. The combination of this decreased sediment supply and a considerable subsidence due to natural compaction and groundwater pumping of 0.75m in 20 years have caused severe coastal erosion at this site.

In this study the available data provided by the other CENAS partners were evaluated and integrated into a quantitative analysis of the sediment transport mechanisms and the impact on the coastal morphology. For the evaluation of the effects of relative sea level changes on the coastal morphology, a baseline study of the present coastal sediment balance was carried out. This baseline study considers the nearshore sediment transport, the relative sea level rise, the sediment input from rivers and nourishments. Based on this study, DHI's sediment transport modelling system LITPACK was calibrated and applied to estimate future coastline evolution for different subsidence schemes.

Summary and conclusions

A morphological baseline study was performed for 3 coastal sites along the Romagna coast, Italy. A quantitative analysis of the observed coastline changes was performed and estimates of the annual erosion/sedimentation rates were derived from this analysis. The longshore variation of the littoral transport rates was calculated on the basis of the observed volumetric changes, the subsidence rates and the estimated fluvial sediment supplies. The results of the baseline study have been applied in the calibration of the sediment transport model which was used to simulate the future coastline evolution for the present sites. In these simulations, the virtual loss of sediment due to the subsidence was included by means of a sink term in the coastal sediment balance. The calibrated coastline evolution model was used to study the coastline development for different subsidence scenarios which were supplied by other CENAS partners.

The present work has shown that the subsidence is a main factor in the coastal sediment balance. Its importance is of the same order of magnitude as the littoral transport and the sediment input from rivers. The simulations of the coastline evolution have shown that the coastal erosion, which is observed at almost all investigated sites, can be reduced considerably, or even disappear completely if no subsidence occurred. It must be stressed though that the subsidence mainly causes coastal problems on a local scale. The general pattern of coastal erosion in the region must be subscribed to the reduced sediment supply from the rivers. The establishment of schemes of detached offshore breakwaters has reduced the coastline retreat locally. In the lee side of these structures, the problems have become worse due to the lack of sediment supply from the littoral currents. This type of coastal protection is not recommended by DHI because it translates the coastal problems rather than solves them. A sustainable coastal protection can be achieved by means of few hard structures which allow for relatively long and exposed beaches, supplemented with additional nourishments.

Presentation author: Andrea Pellizzon